The Most Dangerous (Man-Made) Lava Flow

The surface of an experimentally made corium lava flow. Image: from Journeau et al. (2003), Nuclear Engineering and Design. One of the things I enjoy most is running across fascinating information when I'm not even looking for it. Case in point, today's subject. I was doing some research for my class on Fukushima Dai'ichi and Chernobyl when I ran into some references to lava. "Lava?" I thought, "Why are they talking about lava when I thought I was trying to find out about nuclear accidents?" Lo and behold, what do I find but an entire research field that has been making manmade lava for decades. Sure, we've seen some of the recent manmade lava flows done at Syracuse University and small-scale lava in experiments for some time, but here I was finding research that involved a ton (literally) of manmade lava ... and moreover, these lava have been made by accident on a number of occasions with tragic consequences. Let's back up a bit. What I'm talking about here is the result of a meltdown in the core of a nuclear reactor. This is when the nuclear fission reaction occurring within a nuclear reactor is no longer cooled and contained sufficiently to prevent heating of the rods, cases, core containment vessel and anything else nearby, including the concrete floor of the reactor building. When a meltdown begins to occur, as what happened at Chernobyl in 1986 or Fukushima Dai'ichi in 2011, the ability to cool the reactor is insufficient to keep the fuel rods cool, so heat begins to build -- and build rapidly. The two most important primary isotopes used in nuclear fission reactions are uranium-235 and plutonium-239, so it is their fission caused by the absorbtion of a neutron into isotopes with even shorter half-lives (like cesium and strontium) are what produces the heat in the nuclear reactor core. The chain reaction of fission, decays and absorption of the released alpha particles by other atoms is allowed to go unfettered, the heat will build to the point where the fuel rods (made mostly of enriched U, meaning it has more ^235U than the natural distribution of ^235U) will start to bend and, if the heating is allowed to continue, melt. This is usually controlled by cooling water and control rods that can absorb some of neutrons created by fission and decay. However, if there is a problem, the heat can continues to rise and the fuel rods can become fully molten, that is the "meltdown". So, in a sense, a meltdown in a nuclear reactor is the accidental production of lava.